How many volts to the A/C coil?

[curtis73]

I was wiring up an AC idle solenoid on the 73 Impala Wagon the other day and noticed something. I tested the wires to make sure the I had the right one and noticed that it was only giving me 10v. Battery tested at 12.4v. I didn't think much of it assuming it was designed that way and there was a resistor somewhere. I would leave it alone since the AC works fine, except the 10v isn't enough to make the solenoid open the throttle. It clicks on, but doesn't have the oomph to open the blades.

Is that wire supposed to be 10v, or should it be full voltage?

[sierrap615]

i don't know specifly for your impala, but this is my general suggestion.

confirm the throttle plate isn't sticking.

when you got the 10 volt reading, was it a voltage drop test with the circuit fully connected and energized? if so, check the voltage drop on the positive side and ground side.

if you tested with the circuit opened, try a voltage drop test and see what you find(backprobing may be nessacery)

if you don't find any resistance in the wiring, try powering the solenoid directly from the battery and see if it has the oomph then.

[curtis73]

Originally Posted by sierrap615

confirm the throttle plate isn't sticking.

Check.

when you got the 10 volt reading, was it a voltage drop test with the circuit fully connected and energized? if so, check the voltage drop on the positive side and ground side.

I didn't do that exactly, but I did test continuity from the case of the compressor (where it grounds itself) to the battery negative and came up with zero ohms. I tested the wire before and after the wiring harness, circuit closed and open and got the 10v always. My only guess is that somewhere inside the dash controls the switch is grimy or corroded. Pulling it is a royal pain, but if it needs to be done I can.

if you tested with the circuit opened, try a voltage drop test and see what you find(backprobing may be nessacery)

I'll have to check with my wife first. She's not a big fan of that Sorry, couldn't resist.

Thanks for the help. I'll do what you suggested and try out some other things.

[sierrap615]

n/p - in my experence, eventhough its not much, there is only one, very certain way to measure voltage. last summer i took the easy, fast, and wrong way to measure power to a relay, my DVOM measured a full 12.5 volts, but the relay wouldn't energize. about three days later i finally ohmed out the ground side and got 150,000 ohms, yeah i felt dumb.

[curtis73]

Very good point that I hadn't thought of! Its possible that the tiny load the volt meter puts on it doesn't accurately reflect the resistance that could be there with 15 amps getting sucked through. Thanks for the help

[sierrap615]

curtis, i was looking in some of the old service manuals at school on friday and couldn't find anything about a AC idle solenoid. is this a stock system? can you give me a little more background info? i might be able to find a wiring diagram.

[curtis73]

Sure. GM used two different types of mechanical idle control for two different purposes. The first, found on HD vehicles, early Qjets (through 76 or so), and some non-AC or hot climate vehicles was an anti-dieseling solenoid. This early type was a vacuum brake. During idle it activated holding the throttle open and it was this that actually set idle speed. When you turned off the ignition, no vacuum, and therefore it let the throttle blades close preventing run-on. This is not to be confused with the vacuum pot used to prevent a backfire or stalling when you take your foot off the pedal. That was a vacuum brake located on the rear passenger side of Qjets, or on the front of Carters, where as all of the idle control devices I'm talking about are located on the front driver's side.

The second type was an electro-magnet style. Whenever you give it 12v, it pops out. This could be used as an anti-dieseling solenoid if you gave it power from the ignition lead, or in my case, an AC idle compensator when wired to the AC.

Its an entirely stock system, or at least stock components. The AC system is entirely the original R12 system. The carb is a later Qjet which had a different mounting for the solenoid. My original setup used the solenoid as an anti-dieseling thing since it was originally a New Mexico car, even though it was equipped with AC, it was wired with ignition voltage. I sourced a solenoid to fit the newer Qjet from a caddy with AC. I verified that the wire to it came from the caddy's AC to make sure it was intended as an AC solenoid, but later research proved that it was the same part number regardless of its application... which makes sense. They do the same thing just at different times.

[curtis73]

Here is a little photo of one. Its actually designed for an Edelbrock squarebore carb, so the bracket is different, but the solenoid looks very similar.

[sierrap615]

thanks, i'll see if i can find anything on tuesday.

come to think about it, i do think i saw a anti-desieling relay, i just ignored it because i thought it was used to turn the A/C clutch on during shutdown to prevent desieling by adding a extra load to the engine, which i do beleave some carburated engines have.

[sierrap615]

checked 3 times, found nothing about a A/C Idle controller

[curtis73]

At this point I think I'll leave it as-is. I'm getting full voltage to the dash controls, full voltage out of the dash controls, then the green wire in question disappears into one heck of a wire loom under the dash. When it emerges from the firewall it only has about 10v, so something under there (either intentionally or not) is causing resistance.

It at least works OK, so for now I'll leave well enough alone until I have the time to really tear it apart. Thanks for the help!